To test a transformer, obtain an oscillator and multimeter, sync the measuring devices, and connect the oscillator to the transformer to display a voltage reading. Equipped with the multimeter, use the oscillator to test AC signals of the transformer's wires to determine if the voltage is the appropriate reading for the transformer to operate correctly. 

1. Prepare the devices

Disconnect the transformer from any sources of power or electricity. Turn on the oscillator, and set the frequency to 60 hertz. Set the multimeter to produce results in AC volts, and touch the tips of the multimeter's probe to the output connector of the oscillator.

2. Secure the settings

Adjust the oscillator's signal to 5 volts while it is still connected to the multimeter. Once the reading is at 5 AC volts, disconnect the multimeter from the oscillator. Locate the two primary wires of the transformer, and connect the device to the output area of the oscillator.

3. Test the transformer

To test the transformer, touch the multimeter's probe tips to the transformer's two primary wires while it is connected to the oscillator. The voltage reading should display on the multimeter at this point. A transformer in working order produces a reading varying from 1 to 5 volts.